1. Field of the Invention
This invention relates to variable focal length objectives, and more particularly to compact variable focal length objectives having refractive index distribution type lenses.
2. Description of the Related Art
Recently, the requirement of reducing the bulk and size of the variable focal length objective has been becoming more and more urgent, and there are various proposal for shortening the optical total length.
But, in general, as the optical total length of the variable focal length objective shortens, the Petzval sum is caused to rapidly increase in the negative sense with the result of a large over-correction of the curvature of field. This aberration is very difficult to correct, giving rise to a fatal obstruction in attaining a desired decrease of the optical total length of the variable focal length objective.
In order to achieve an advance in the compactness of the entire system of the variable focal length objective, for example, of the type comprising a plurality of lens units, of which the first and second counting from the front are positive and negative in power respectively and movable with the air separation therebetween being made variable for varying the focal length of the entire system, or of the type comprising, from front to rear, a positive first lens unit, a negative second lens unit, a positive or negative third lens unit and a positive fourth lens unit, the first to third lens units constituting a zoom section, and the fourth lens unit constituting a relay lens, in other words, the so-called 4-component type, or of the type comprising a plurality of lens units, of from front to rear, a positive first lens unit, a negative second lens unit and a third lens unit of strong positive power, whereby as the focal length of the entire system varies from the shortest to the longest, whilst the separation between the first and second lens units increases and the separation between the second and third lens units decreases, the third lens unit moves forward, there are the three conventional methods (1) by strengthening the power of each of the lens units which constitute the zoom section, (2) by decreasing the telephoto ratio of the relay lens, and (3) by employing the telephoto type in designing the third positive lens unit. However, the use of the first method (1) results in the production of a large negative value of the Petzval sum in the second lens unit as this lens unit usually as the variator has the strongest power. This implies that the curvature of field is extremely over-corrected. The second method (2), too, has a similar result, becuase the direction of decreasing the telephoto ratio of the relay lens coincides with the direction in which the Petzval sum is produced to a negative value. So, the use of this method unavoidably leads to over-correct the curvature of field. Also, the third method (3) is not useful from the similar reason.
Further, if such Petzval sum is corrected by lowering the refractive index of the convex lens, or by using a positive lens of strong power in combination with a negative lens, very large spherical aberration or very large higher order aberrations is or are produced which cannot be well corrected. In such a manner, the requirement of reducing the bulk and size of the variable focal length objective is in contradiction to the requirement of correcting the Petzval sum, as far as the system of the lenses all of which are made of homogeneous material is concerned. This is valid not only in the above-mentioned types of variable focal length objectives, but also another types of variable focal length objectives in which as the first lens unit moves during zooming, the total length increases with increase in the focal length, or in which the fourth lens unit is made axially movable with zooming.
Further, the 2-component zoom lenses comprising from front to rear a negative first lens unit and a positive second lens unit, are of no exception.
That is, in the case of the 2-component zoom lenses, letting the focal lengths of the first and second lens units be denoted by f1 and f2 respectively and the image magnification of the second lens unit by .beta.2, the focal length f of the entire system can be expressed by f=f1.times..beta.2, and the distance S2 between the object side position and the image side position of the second lens unit by S2=f2.times.(1-.beta.2).sup.2 /.beta.2. Therefore, even with the same image magnification of the second lens unit, the shorter the focal length of the second lens unit, the greater the reduction of the optical total length of the entire system can be made. It should be noted here that to allow for a reduction of the focal length of the second lens unit with maintenance of the air separation between the first and second lens units at the telephoto end to larger values than an acceptable minimum, the second lens unit must be designated to the telephoto type of power arrangement, and the tendency of the telephoto type must be so strengthened that the principal point is shifted enough forward. The use of this method in achieving the desired reduction of the total length results in the production of a large negative value of the Petzval sum. So the curvature of field is over-corrected. Also, the correction of the Petzval sum by lowering the refractive index of the convex lens, or by using a positive lens of strong power in combination with a negative lens of strong power, results in objectionable increase of the spherical aberration or the production of higher order aberrations, any of which is difficult to correct.
In such a manner, regardless of what type is employed in designing variable focal length objectives, attempts to a desired reduction of the total length of the entire system have been foiled without exception by the difficulty of well correcting the curvature of field.